It is well known in the art that the temperature of an electric device can affect its performance and service life. Temperature can be an especially significant factor when the electrical device is a battery.
To ensure the reliable functioning of a backup power source which comprises a battery, the battery must be replaced relatively frequently, regardless of the number of load cycles imposed upon the battery, to assure that backup power will be available when required. This periodic replacement is particularly required when the backup battery is used in a time-of-use power meter. These power meters (which, more accurately, are watt-hour meters) are typically mounted on the side of buildings, where they are exposed to sunlight. High battery temperatures resulting from heating by solar irradiation reduces the service life of the installed batteries, necessitating monitoring or replacement of the batteries at more frequent intervals than would be required if the batteries were kept cool.
Time-of-use (TOU) power meters measure the amount of power delivered and they also contain circuitry which records power consumption as a function of the time of day and day of the week. The TOU power meter thus allows utility companies to adopt rate structures made up of different rates for power used during particular periods of the day or on particular days of the week. Batteries are typically used to provide back-up power in the event that power from the circuitry's normal power supply, the utility line, is interrupted for some reason, e.g. due to a lightning strike. Maintaining operation of the clock function during a power outage is critical for accurate resumption of TOU power metering when power is restored on the utility line.
Batteries that are used for such purposes must be reliable and have a long service life in order to minimize the administrative burden, labor and economic expense of replacing them. Some batteries, such as solid-state lithium-iodine type batteries, have a service life in excess of twenty years if their temperature is maintained in the range of approximately 10.degree. C.-37.degree. C. The longer the battery is exposed to temperatures above that range, however, the shorter becomes the service life of the battery, and its power output may become unreliable. For example, in many southwestern parts of the United States the average yearly temperature is approximately 20.degree. C. (e.g. in Phoenix, Ariz.), i.e. well within the permissible temperature range. However, temperature extremes on many days can rise above that range. Further, solar irradiation in those areas can be of an intensity to cause a rise in battery temperature above ambient air temperature. Such a temperature rise, when combined with a high ambient temperature, creates conditions which can have a deleterious effect on a backup power supply battery exposed to such conditions.
To avoid battery failure or deterioration, a number of techniques have been suggested to compensate for this undesirable heating. It has already been proposed to use a thermoelectric module, powered from the utility line, to cool a battery in a TOU power meter. Such an arrangement is disclosed and claimed in U.S. Pat. No. 4,999,576 of L. Levinson, which is assigned to the present assignee. As the primary source of undesired battery heating in an externally mounted TOU power meter is solar irradiation, there is typically no need to have the cooling apparatus in operation in the absence of such heating. Consequently, in systems that use thermoelectric cooling modules powered from a utility line, special control circuitry is used to monitor battery temperature and to activate the cooling module only when the temperature rises above a selected level. In addition, the module powered from the utility line is unable to cool the battery during a power outage. A line-powered thermoelectric module also requires power supply equipment to step down the line voltage and convert the AC to DC. The added complexity of this control and power supply equipment, plus the additional maintenance requirements associated with it, may reduce the commercial attractiveness of such a cooling system.
Another technique suggested to assist battery cooling is a passive cooling system utilizing a chimney effect to generate an air flow across a heat exchanger thermally coupled with the battery. Such an arrangement is disclosed and claimed in L. Levinson et al. co-pending application Ser. No. 07/444,338, filed 1 Nov. 1989, now U.S. Pat. No. 5,035,964, and continuation-in-part application Ser. No. 07/699,085, filed May 13, 1991 which are assigned to the assignee of the present invention. The passive cooling system relies on solar irradiation to heat air in a lower part of the chimney. As the air ascends in the chimney, it produces an air flow across the heat exchanger surfaces located within the chimney. This arrangement uses air at a temperature above ambient temperature as the cooling medium, and thus may not provide as immediate and as effective cooling of the battery as a thermoelectric module.